Electrolytic manufacture of alkyl-substituted hydroquinones

ABSTRACT

Alkylhydroquinones with from 1 to 3 alkyl radicals, which can contain from 1 to 4 carbon atoms, are produced by electrolytic oxidation of the corresponding alkylphenols which are unsubstituted in the para-position to the hydroxyl group, followed by electrolytic reduction of the resulting alkylquinones, wherein the electrolysis is carried out in the presence of a non-oxidizing aqueous mineral acid and in the presence of a water-soluble ketone.

United States Patent Degner [4 1 Sept. 30, 1975 ELECTROLYTIC MANUFACTUREOF ALKYL-SUBSTITUTED HYDROQUINONES Inventor: Dieter Degner,Ludwigshafen,

Germany BASF Aktiengesellschaft, Ludwigshafen, Rhine, Germany Filed:Dec. 4, 1974 Appl. N0.: 529,284

Assignee:

Foreign Application Priority Data Dec. 5, 1973 Germany 2360494 US. Cl204/73; 204/78 C25B 3/02; C25B 3/04 Field of Search 204/72, 73, 78

References Cited UNITED STATES PATENTS 10/1971 Covitz et a] 204/783,721,615 3/1973 Fremery et a1 204/73 R Primary Examiner-R. L. AndrewsAttorney, Agent, or FirmJohnston, Keil, Thompson & Shurtleff [5 7ABSTRACT 9 Claims, No Drawings ELECTROLYTIC MANUFACTURE OFALKYL-SUBSTITUTED HYDROQUINONES This application discloses and claimssubject matter described in German Pat. application No. P 23 60 494.6,filed Dec. 5, 1973, which is incorporated herein by reference.

The invention relates to a process for the manufacture ofalkylhydroquinones by electrolytic oxidation of alkylphenols andsubsequent electrolytic reduction of the quinones thus obtained.

The anodic oxidation of phenols to the corresponding benzoquinones in acompartmented cell; and the electrochemical synthesis of hydroquinonesin a noncompartmented cell, have been known for a considerable time (cf.Berichte der deutschen Chemischen Gesellschaft, Vol. 47, p. 2,003(1914), Helv. Chim. Acta, Vol. 2, p. 583 (1919), Vol. 8, p. 74(1925),Vol.10, p. 40 1927) and Vol. 10, p. 102 (1927). However, in this processneither the product yield nor the current efficiency are very high, andfurthermore a number of un desired byproducts are formed. Theseby-products necessitate expensive purification operations.

German Published application No. 1,643,558 has disclosed convertingunsubstituted phenol into hydroquinone in 90 per cent yield throughappropriate choice of the electrolysis parameterssuch as currentdensity, depolarizer and electrolyte concentration. The currentefficiencies are above 60%. If attempts are made to apply theseconditions to the anodic oxidation of alkylsubstituted phenols, thecorresponding hydroquinones are obtained, but the yields are very lowand the current efficiencies even drop below Further, ChemicalCommunications 1971, pp. 1,643, et seq. discloses that dimethylphenolcan be oxidized electrolytically to dimethylquinone in the presence ofacetonitrile as the solvent. However, if attempts are made to reduce analkylquinone, thus obtained, to the corresponding hydroquinone, theyields obtained leave much to be desired. An essential disadvantage isthat the hydroquinones obtained are insufficiently pure for furtherconversion and thus necessitate an expensive purification process. I

I have found that alkylhydroquinones of the general formula in which Ris alkyl of l to 4 carbon atoms and n is an integer from 1 to 3 can bemanufactured particularly advantageously in an electrolyticprocesswherein alkylphenols of the formula group, are oxidized anodically in anon-oxidizing aqueous mineral acid in the presence of a watersolubleketone and the reaction mixture containing the correspondingalkylquinones is reduced cathodically.

It is an advantage of the new process that both the yields and thecurrent efficiences are good. A decisive advantage of the new process isthat the hydroquinones as obtained are more than 90 per cent pure andrequire no additional purification before further processing.

In the preferred alkylphenols of the formula II, R is methyl. Thepara-position, where oxidation is to take place, in the initial phenolII must of course always be unsubstituted. Examples of suitable phenolsare 2- methylphenol, 2,6-dimethylphenol, 2,3,6- trimethylphenol and2,3,5-trimethylphenol.

2,6-Dimethylphenol and 2,3,6-trimethylphenol have acquired particularlygreat importance in industry.

The electrolysis is carried out in the presence of an aqueous solutionof a non-oxidizing mineral acid, especially sulfuric acid. In general,the concentration of the mineral acid used is from 1 to 20 percent byweight, especially from 5 to 10 percent by weight.

During the anodic oxidation and the cathodic reduction, the temperatureis maintained at its ambient value or slightly above, for example atfrom 20 to 40C. Under no circumstances should the boiling points of thesolvents used be exceeded.

It is advantageous to maintain current densities greater than 5 amperesper dm during the electrolysis. In general, the range of currentdensities used is from 5 to 20 amperes per dm The anodes used are leaddioxide, or electrodes coated with lead dioxide, or electrodes of noblemetals, such as, for example, platinum, platinized titanium or gold.Lead dioxide anodes are preferred.

Cathodes which can be used are lead, mercury, cadmium, tin, zinc,copper, nickel, silver amalgam and lead amalgam electrodes. Leadelectrodes have acquired particular importance.

The anodic oxidation of the phenols is preferably carried out in acompartmented anode chamber and the alkylquinones obtained are thenreduced in a subsequent stage, in a cathode chamber which is alsocompartmented. It has proved particularly convenient to use acompartmented cell, carry out the oxidation of the alkylphenols II inthe anode chamber, pass the solution thus obtained into the cathodechamber and there carry out the electrochemical reduction to thecorresponding hydroquinone.

An essential feature of the invention is the use of a water-solubleketone as the solvent. Examples of suitable water-soluble ketones areacetone, methyl ethyl ketone and diethyl ketone. Acetone has proved aparticularly suitable solvent. The electrolysis mixture usedadvantageously contains from 20 to percent by weight, especially from 40to 60 percent by weight, of acetone.

The initial electrolysis mixture preferably contains from 1 to 10percent by weight of alkylphenols of the formula II.

The alkylhydroquinones obtained as end products are generally isolatedby evaporating off the solvent. They can also be extracted with asuitable waterimmiscible solvent and be isolated therefrom byconventional methods, for example precipitation or fractionalcrystallization.

Alkyl-substituted hydroquinones manufactured by the process of theinvention can be used for the manufacture of plant protection agents,dyes or biologically active materials, for example vitamin E (cf. S.F.Dyke; On completion of the electrolysis, the catholyte is The Chemistryof the Vitamins, pp. 256 et seq., Interworked up analogously to Example2. This gives trimescience Publishers 1965). thylhydroquinone which isover 90 percent pure. The Alkylhydroquinones can also be used aspolymeriy l (based ri y p Converted) is zation inhibitors (cf. BelgianPat. No. 779,388). 774%- The Examples which follow illustrate theprocess of COMPARATIVE EXAMPLE the invention.

If the procedure described in Examples 3 and 4 is fol- EXAMPLE 1 lowedbut acetonitrile is used instead of acetone, the I 10 yield of2,3,6-trimethylhydroquinone is 45 55% and Anodic oxidation of2,6dimethylphenol. the. purity is only 70 75%. Apparatus: compartmentedcell with Cation exchange membrane w l Anode: PbO electrode; surfacearea: 0.66 dm e C Anolyte: 24.4 g (0.2 mole) of 2,6-dimethylphenol 1. Aprocess for the electrochemical manufacture of 550 ml of H20 450 ml ofacetone an alkylhydroquinone of the general formula I 49 g ofconcentrated H2804 Catholyte: IN H2504 Cathode: Pb electrode Charge O:0.8 F Y Current l: l0 A OH 1 On completion of the electrolysis, a sampleof the an- (R olyte was taken, the. acetone was distilled off and theresidue was repeatedly extracted with ether. After dis- I OH tilling offthe ether, the 2,6-dimethyl-p-benzoquinone and unconverted startingmaterial are determined by gas chromatography. According to thesedeterminations, the yield of 2,6-dimethyl-o-benzoquinone (based in whichR is alkyl of l to 4 carbon atoms and n is an on 2,6-dimethylphenolconverted) is 86.2% and the n g from 1 t0 wherein an lkylph n f h gncurrent efficiency is 54.8%. eral formula EXAMPLE 2 Electrochemicalsynthesis of 2.6-dimethylhydroquinone 0H Apparatus Anode Anolyte As inExample I II Cathode n l and Q Catholyte; the unolyte from Example 1.

Af o lt' fth ltrlsis,theat c mp e [on o e ec O y Ce one Is in which Rand n have the above meanings, and which distilled off and the residueis steam-distilled to remove 1S unsubstituted in the para-position tothe hydroxyl unconverted 2,6-dimethylphenol. On cooling the resi- 40 g pis Oxidized anodicany in a nomoxidizing q duefiom h Steam dlsnn'fmoniz'dlmethylhydroqul' ous mineral acid and in the presence of awater-soluble f preclpltates' The yled based on ketone, and the reactionmixture containing the corredimethylphenol converted. The2,6-dimethylhydroquisponding alkylquinone is reduced cathodicany obtamed90% pure 2. a process as claimed in claim 1, wherein the anodic EXAMPLE3 5 oxidation and the cathodic reduction are carried out at roomtemperature or at a temperature of up to 40C. 3. A process as claimed inclaim 1, wherein sulfuric Anodic oxidation of 2,3,6-trimethylphenolApparatus: compartmented cellowgthzatzion exchange membrane acid of from1 to 20 percent strength by weight iS used 1123 i i iii i rfw i ei er2,3,6- trimethyIphenQI as the non'oxldlzmg aque9us mfneral 500 ml of H204. A process as claimed in claim 1, wherein a current :3 0f d H 0density of 5 to 20 amperes per dm is maintained during catholyte: i z gjC Is the electrolysis. I I Cathode: Pb 5. A process as claimed in claim1, wherein acetone, QI R115 methyl ethyl ketone or diethyl ketone isused as the water-soluble ketone; on Working p a sample of the materialleaving the 6. A process as claimed in claim 1, wherein theelecelectrolysis, analogously to Example 1, y -P' trolysis mixturecontains from 20 to percent by benzoquinone is obtained in 84.6 percentyield (based i h f acetone 2,3,64rimethylphen0l Converted) The current7. A process as claimed in claim 1, wherein the elecciency is 54%. 60trolysis mixture contains from 1 to 10 percent by weight of alkylphenol.

8. A process as claimed in claim 1, wherein an alkylphenol of theformula II, in which R is methyl, is used.

9. A process as claimed in claim 1, wherein 2- EXAMPLE 4 Electrochemicalsynthesis of trimethylhydroquinone Apparatus v Anode methylphenol,2,6-dimethylphenol, 2,3,6- Amlyte As Example 3 trimethylphenol or2,3,5-trimethylphenol is used as the Cathode I and Q alkylphenol.

Catholyte: the anolyte from Example 3. =i

1. A PROCESS FOR THE ELCTROCHEMICAL MANUFACTURE OF AN ALKYLHYDROQUINONEOF THE GENERAL FORMULA
 2. a process as claimed in claim 1, wherein theanodic oxidation and the cathodic reduction are carried out at roomtemperature or at a temperature of up to 40*C.
 3. A process as claimedin claim 1, wherein sulfuric acid of from 1 to 20 percent strength byweight is used as the non-oxidizing aqueous mineral acid.
 4. A processas claimed in claim 1, wherein a current density of 5 to 20 amperes perdm2 is maintained during the electrolysis.
 5. A process as claimed inclaim 1, wherein acetone, methyl ethyl ketone or diethyl ketone is usedas the water-soluble ketone.
 6. A process as claimed in claim 1, whereinthe electrolysis mixture contains from 20 to 60 percent by weight ofacetone.
 7. A process as claimed in claim 1, wherein the electrolysismixture contains from 1 to 10 percent by weight of alkylphenol.
 8. Aprocess as claimed in claim 1, wherein an alkylphenol of the formula II,in which R is methyl, is used.
 9. A process as claimed in claim 1,wherein 2-methylphenol, 2,6-dimethylphenol, 2,3,6-trimethylphenol or2,3,5-trimethylphenol is used as the alkylphenol.